A TDD system typically refers to a communication system in which uplink and downlink transmissions would share a single carrier frequency but be divided in the time domain across different subframes. In a typical Long Term Evolution (LTE) communication system, a radio frame would be divided into 10 subframes, and each subframe could be allocated for an uplink transmission, a downlink transmission, or a special subframe which is used as a guard period and/or as a time slot reserved for a pilot signal. Such allocation schemes for each individual subframes could be defined according to several possible configurations.
FIG. 1 is a diagram which illustrates TDD uplink-downlink frame configurations in a conventional LTE communication system with a D denoting a downlink subframe, a U denoting a uplink subframe, or a S denoting a special subframe for each of the subframes numbered from 0 to 9. For example, according to the diagram in FIG. 1, if the uplink-downlink frame configuration zero is selected, then subframe numbers 0 and 5 would be allocated for downlink transmissions, subframe numbers 1 and 6 would be allocated as special subframes, and the rest of the subframes, subframe numbers 2˜4 and 7˜9, would be allocated for uplink transmissions. The uplink to downlink ratio for configuration 0 would be 2 versus 6.
In order to effectively increase data rates in LTE/LTE-A and future generations of broadband wireless communication systems, Carrier Aggregation (CA) could be an effective way to increase the data rates. Carrier aggregation could be used in both a Frequency Domain Duplex system (FDD) and a Time Domain Duplex system (TDD) to combine frequency bandwidths in order to increase the capacity of a communication system. For the current Long Term Evolution Advanced (LTE-A) system as an example, each aggregated carrier is called a component carrier and has a bandwidth of 1.4, 3, 5, 10, 15 or 20 MHz. Since a maximum of five component carriers could be aggregated under LTE-A, a total of 100 MHz of maximum bandwidths could be provided under the carrier aggregation scheme. Each component carrier could also have different bandwidths. Two component carriers could be frequency contiguous or adjacent to each other, but any two component carriers not frequency continuous to each other may also be aggregated. Also for each component carrier, a time division duplex (TDD) scheme in which uplink and downlink transmissions would share a single carrier frequency but be divided in the time domain across different subframes could be imposed.
When carrier aggregation is utilized by a wireless communication system, each component carrier could be considered to serve an individual cell. Each cell may have a different coverage range or may overlap with another cell partially or completely. When carriers are aggregated, each carrier is referred to as a component carrier. A component carrier could be categorized into one of two categories—a primary component carrier and a secondary component carrier. The primary component carrier would be the main carrier within a coverage area, and thus there would be a primary downlink carrier and an associated uplink primary component carrier. Additionally, there could also be one or more secondary component carriers. The primary component carrier would serve the primary serving cell (PCC) and could provide most or all of the signaling transmissions for both uplinks and downlinks. Each secondary component carrier would serve a secondary serving cell (SSC) for downlinks and possibly uplinks and would mostly be used carry user data.
The use of primary component carriers and secondary component carriers in a carrier aggregation operation could be seen in heterogeneous wireless network deployment scenarios in which some cells with greater transmission ranges such as macrocells could provide the primary component carriers while other cells with local coverage such as small cells or femtocells would provide the secondary component carriers in order to increase the data transmitting capacities. In a dual-connectivity case, a user device might connect to both a macrocell base station and a small cell base station to enjoy both the network coverage and higher capacity. In one example, a dual-connecting UE might be served by a coverage carrier by a macrocell base station and a capacity carrier by small cell base station.
However, the configuration of downlink subframes and uplink subframes have conventionally been quite static during system operations since a network operator would select a configuration based on the long-term average of uplink and downlink traffic ratios. It has been observed recently that wireless data traffic has becoming bursty in nature, and variations of downlink-uplink traffic ratios could be at times very fast changing. Consequently, a dynamic TDD system in which uplink and downlink subframe ratios could be adaptively configured according to instantaneous traffic conditions has been considered in order to improve the performance of a communication system as “Further Enhancements to LTE TDD for DL-UL Interference Management and Traffic Adaptation” has been considered to be an important working item for 3GPP Release 12.
Furthermore, the traditional SIB update mechanism is not yet satisfactory for the purpose of dynamically updating a system parameter such as the uplink-downlink frame configuration in a real time basis. The system information could be broadcasted, for example, every 320 milliseconds. The broadcast periodicity is kept relatively short in order to accommodate UEs which may frequently move in and out of the broadcast range without having to wait for a long period to acquire system information.
One problem is that a base station cannot make alterations to system information during every broadcast as it would mean that the UEs have to check whether the system information is altered more frequently than necessary. Instead, a base station may only modify system information at the front boundary of a modification period (MP), which may occur, for example, every 40 seconds. As the result of the long modification period, it would be rather difficult for a base station to instantaneously change the uplink-downlink frame configuration in case the traffic becomes heavy all in a sudden.
As the wireless communication traffic may become relatively non-existent, energy savings and interference reductions could be achieved when communications are turned off in certain subframes. The potentially bursty nature of wireless communication traffic may nevertheless require a system to dynamically activate and set dormant certain downlink or uplink subframes. Therefore, a mechanism would be needed to provide a solution for configurations for dynamic activation of radio resources. In order to achieve the goal of dynamically adjusting subframe configurations, signaling mechanism would be essential as signaling mechanisms would communicated among network control nodes, base stations, and UEs. Without a proper signaling mechanism for subframe configurations, a base station would either be overloaded under heavy data traffic or lightly loaded when continuously receiving empty subframes. A user equipment may also benefit by saving computational power and energy consumptions under a properly designed signaling mechanism for subframe configurations.
Therefore, the present disclosure proposes a design which provides flexibilities in a network system operation to dynamically meet various traffic demands and interference conditions.